1. Field of the Invention
This invention relates to a large scale process for the manufacture of rice wine and particularly a rice vinegar having the attributes of ancient jar prepared vinegar. A single autoclave, fermentation vessel is used to carry out process steps including washing and soaking, steam cooking, high temperature liquification, saccharification and fermentation until an ethanol concentration of 18% is reached, and the vessel includes bottom scrapers which ensure a more complete gelatinization, liquification and sterilization of the rice. Two strains of Aspergillus oryzae are grown on steamed wheat and utilized as a source of enzymes to provide a unique, characteristic flavor, aroma and umami to the mash. One preferred method step includes two separate additions of alpha-amylase to the mash to promote gelatinization and liquification and ensure that the mash has a starch content of at least 30%.
2. Description of the Prior Art
Vinegar, as the term is known today, may be prepared from almost any watery substance that contains sugar and other nutrients to provide an alcoholic fermentation which is followed by acetification. Primarily, vinegar is a dilute solution of acetic acid but may also include many unaltered soluble ingredients from which it is made as well as fermentation products other than acetic acid. Although vinegars can be prepared from any of several alcoholic products, the character, including the flavor and aroma, of any particular vinegar is dependent upon the character of the substance from which it is fermented.
In the United States, much of the better quality vinegar used is prepared from fermented apple juice; however, wine vinegar is manufactured in many of the wine-producing areas of the world and in some areas, popular vinegars are produced from the juices of pineapples, oranges, persimmons, plums and other fruits. Use of malt vinegars is widespread in countries such as England, while tea beverage vinegars are popular in certain areas including Indonesia and Russia.
Vinegar may be used for a number of various purposes. Oftentimes, vinegar is used without alteration or with addition of simple herbs as a condiment or dressing. Vinegars are also used extensively for the preparation of sauces such as ketchup, fruit sauces, savory sauces, mayonnaise and salad dressings. Vinegar is a main constituent of many relishes, chutneys and piccalilli. Furthermore, vinegar is of major importance to industry in the manufacture and processing of brined vegetables such as cauliflower, onions and the like and for making various types of pickles.
In ancient China, vinegars were mostly produced from rice, since rice was one of the major crops of the area. Many Orientals consume rice vinegar on a daily basis for health or medicinal reasons; in addition, it is used as an important ingredient for soft drinks, fruit juices, and other beverages as a substitute for citric acid. While rice vinegar may be inexpensively manufactured from rice wine residue or sake residue, there has been an increasing consumer preference for vinegar prepared from a rice mash which provides its own characteristic flavor, aroma, and umami, the latter word being a Japanese term roughly defined as an intensified flavor and aroma.
Virtually all flavors, including sourness, bitterness, sweetness and saltiness, are found in vinegar due to the many different kinds of flavor and odor molecules which are present. Chemical factors affecting the flavor of vinegar include types of nucleic acids and amino acids (including short peptides) which are present, while aroma is determined largely from fatty acids of varying chain lengths. The color of rice vinegar may be changed according to consumer preference by exposing the product to heat which functions to intensify the color.
In brief, the known methods for producing rice vinegar include the steps of saccharification of rice which is followed by alcoholic fermentation and then acetification. Additionally, a quantity of koji, or fermented grains, may be added to assist the saccharification process and enchance the flavor of the final product. The koji serves as a source of a number of enzymes that catalyze the degradation of rice to soluble products which provide the substrates for yeast and bacteria in the subsequent fermentation stages. Moreover, koji also produces a number of chemicals which impart a unique flavor and aroma to the finished product.
The traditional ancient method of preparing rice vinegar includes the initial step of polishing the rice with a light force to remove the hull and embryo so that the mycelia of the fungal cultures can easily penetrate and saccharify the rice. Next, the rice is washed to remove residual hulls and soaked in water of good quality. The rice is then steamed to convert starch in the rice into alpha-starch and denature the protein in the rice. The rice is steamed for one hour in a basket placed atop a pot or alternatively may be cooked on a conveyor which exposes the rice to a steam bath for 20 to 30 minutes. The cooked rice must then be cooled to about 20.degree.-25.degree. C. by use of a cooling machine or by manually spreading the rice on cloth for air cooling over an extended period of time.
Typically, koji is prepared by thoroughly mixing an inoculum of koji with steamed rice that is cooled to 30.degree.-32.degree. C. The mixture is spread on a bed in a thin layer, and air at about 32.degree. C. and having a relative humidity of 90-94% is passed through the bed for 20 hours at which time the mixture is turned over. The temperature of the koji will continue to rise during additional fermentation which normally lasts 40 hours.
To begin production of alcohol, yeast inoculum is prepared in a large quantity and koji and steamed rice are typically added three times over a period of four to five days. This process is carried out in an open system without sterilization, and thus complicated techniques are implemented in an effort to discourage invading microorganisms such as wild yeasts and lactic acid bacteria. Fermentation of the rice and koji mash is carried out at an extremely low temperature in the range of 9.degree.-11.degree. C. over a period of 25 to 30 days and as a result is extremely labor intensive.
Typical Chinese vinegar factories even today employ 200 to 700 workers producing only 8000 gallons of vinegar per month. Furthermore, better quality rice vinegar requires at least two to six years to complete the fermentation and acetification of a marketable product.
An upgraded method which is currently in widespread use for preparation of a rice-vinegar mash has been developed whereby the liquification and saccharification of the rice, that is steamed separately outside a fermentor, is carried out utilizing commercially available alpha- and beta-amylases to obtain a rice mash that has a starch content of less than 18%. A small amount, on the order of 10%, of rice-koji may be added to facilitate the saccharification process and enhance the flavor of the final product. The rice mash is press filtered to obtain a clear filtrate to which baker's yeast is added to start ethanol fermentation. A concentration of 10% ethanol is produced during the process which is carried out at a relatively low temperature, in the range of 15.degree.-20.degree. C. The mash is then diluted approximately two times with the addition of food grade pure ethanol for a submerged oxidation of ethanol to acetic acid.
However, the widely used, upgraded method is subject to many shortcomings. Since the mash contains only approximately 10% ethanol, autolysis of the yeast cell is either completely absent or minimal unless the mash is allowed to stand for a few weeks. Unfortunately, putrification of the mash may occur when the mash stands for such a relatively long time. Secondly, due to the lack of proteolytic enzymes in the rice-koji, the amino acid content is extremely low, which adversely affects the flavor and aroma of the finished product.
Ancient Japanese and Chinese techniques are known which allow saccharification, alcoholic fermentation and acetification to be performed in a common vessel. The entire procedure is carried out as a solid fermentation, to permit an extremely high content of starch in the fermentation mash. The quality of the rice vinegar produced by such ancient methods is highly valuable due to the binding of amino acids to acetic acid which results in such a unique, mild, mellow, and smooth flavor that this vinegar can be used not only as a condiment but also as a drink or as an ingredient for a beverage.
In accordance with the Japanese technique, seed vinegar is sprayed on walls of an earthenware, glazed jar. Next, a layer of koji, a layer of steamed rice, and a second layer of koji are placed in the jar and the contents are allowed to ferment at 20.degree.-28.degree. C. for three to six months, often by placing the jars in an open field. Traditionally, a natural flora of yeasts and acetic acid bacteria are utilized, although the fermentation time and the amount of rice required may be reduced by substitution of pure cultures of microorganisms.
In accordance with the traditional Chinese process, a quantity of rice which has been separately steamed is put into glazed jars and koji is made of the cooked rice. Water is then added to complete saccharification of the rice, after which alcoholic fermentation followed by acetification are undertaken in the same vessel. Unfortunately the success rate is rather low because the natural flora, temperature and acidity are generally not controlled in this type of process. It has been suggested that the Chinese process could be improved by control of the temperature, use of pure cultures to enhance saccharification, and addition of water in two stages after which the jar is sealed before inoculation of quality seed vinegar and incubation at 28.degree.-31.degree. C.
However, all of the above known processes for producing ancient quality rice vinegar provide inconsistent results, are labor intensive and in general extremely time consuming. Furthermore, it has been found to be very difficult to repeatably obtain a finished product having a consistently strong and characteristic flavor, aroma and umami.